Process for recovering oil by in situ combustion



Oct. 31, 1967 sMlTH ET AL PROCESS FOR RECOVERING OIL BY IN SITU COMBUSTION Filed 'Jul 28, 1964 INVEN TORS. 54am All) 4770RNEV.

heating of the formation by the United States Patent Office 3,349,847 Patented Oct. 31, 1967 3,349,847 PROCESS FOR RECOVERING OIL BY IN SITU COMBUSTIQN Francis M. Smith, Butler, and Paul L. Terwiiliger, Oakmont, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Filed July 28, 1964, Ser. No. 385,691 13 Claims. (Cl. 16639) This invention relates to a method for recovering oil from an underground rock formation by in situ combustion of oil contained in the formation.

The improvement of oil recovery from a subterranean formation by the combustion in situ of part of the oil has been suggested as a possible solution to many problems arising in oil production. For example, some formations contain such viscous crude oils that the oil cannot be produced economically by conventional techniques. In those formations, in situ combustion of part of the oil heats the formation, reduces-the viscosity of the oil, and generates energy in the form of steam and expanding gases that augments the expulsive forces induced in the formation by the injection of combustion-supporting gas to displace oil from the formation. Other formations contain crude oil having in solution significant amounts of waxy hydrocarbon substances generally known as paraifins. As oil flows through the formation and the well bore, the reduction of temperature in, and the evolution of volatile constituents from, the crude oil cause the precipitation and accumulation of parafiins in the well bore and the surrounding formation. Intermittent in situ combustion of crude oil near the well bore prevents paraifin deposition in or near the well bore owing to the heating of the produced oil as it flows through the heated formation.

When stimulation of oil production from a formation is achieved through periodic in situ combustion of the formation oil near the well bore, the injection of an oxygencontaining gas to support combustion leaves the well bore and the surrounding formation filled with a combustionsupporting gas. Consequently, the danger of a down-hole fire or explosion upon the subsequent production of formation oil into the well bore is imminent, and experience has indicated that, in fact, such accidents are not uncommon in the practice of such methods of well stimulation.

This invention resides in a method of formation heating by in situ combustion of the crude oil whereby the danger of down-hole combustion of oil subsequently produced is precluded. Prior to the production of oil, an oxygen-free gas is injected to displace the oxygen-containing gas from the well bore and the surrounding formation outwardly through the combustion front in the formation.

. The operation of this invention is explained more fully by reference to the specific embodiment described in the accompanying drawings wherein:

FIGURE 1 is a diagrammatic view, partially in crosssection, showing the well apparatus and formation during e period of oil ignition and the early stages of in situ combustion.

' FIGURE 2 is a diagrammatic view, partially in crosssection, showing the well apparatus and formation during the later stages of in situ combustion.

Referring to FIGURE 1, a well bore At a desired level in formation 14, a circumferential rin -shaped notch Although the dimensions and orientation of fracture 22 in formation 14 are determined by the particular characteristics of each individual formation, it is preferred that fracture 22 be a substantially horizontal fracture that is created in the formation by the single-point entry technique for hydraulic fracturing. One suitable method of single-point entry fracturing is described in of Huitt et :al., issued Feb. 18, 1964. To permit substantially uniform heating of formation 14, it is desirable that fracture 22 extend radially from well bore 10 for a distance of about 50 to about 200 feet, and preferably for a distance of about to about feet. Although it is not essential to the effective use of the process of this invention, the reasonably rapid heating of formation 14 is ordinarily desirable and is assured if fracture 22 has a sufficiently high flow capacity. The term fracture flow capacity is defined as the product of the permeability of a propped fracture times the fracture width and, in the context of this application for Letters Patent, a high capacity fracture is one having a flow capacity greater than 10,000 millidarcy-feet. Fracture 22 can be located at any level in formation 14, but it is preferred that fracture 22, be located near the bottom of formation 14 so that subsequent oil production into the fracture is augmented by the effects of gravity drainage.

After fracture 22 is created in formation 14, a suitable plug 24, such as a retriveable bridge plug, is set in casing 16 below notch 20 and approximately adjacent the bottom of formation 14 to isolate the section of well bore 10 above fracture 22 from the section of the well bore below the fracture. Then a string of tubing 26 is run inside casing 16 to a depth a few feet above that of fracture 22, and a fluid fuel burner 28 is suspended in the lower end of tubing 26 from a string of fuel tubing 30. Several types of down-hole fluid fuel burners are available for use with the process of this invention. An example of such a burner is that described in US. Patent No. 2,668,592 of lPiros et al., issued Feb. 9, 1954. Burner 23 shown in FIGURE 1 is merely a diagrammatic representation of the burner and is not intended as a detailed illustration of a particular suitable apparatus.

After burner 28 is inserted in tubing 26, air is injected into the well apparatus downwardly through annuli 4i) and 42 formed, respectively, between fuel tubing 30 and tubing 26 and between tubing 26 and casing 16. The air in annulus 40 enters burner 28 through slots 44 cut in the wall of burner 28 and all of the injected air flows through fracture 22 into formation 14. The injection of air into formation 14 is continued until a desired constant rate of air injection and a substantially constant injection pressure have been established. Next, fuel is injected through fuel tubing 30 while air is injected through annuli 40 and 42. Burner 28 is ignited and the mixture of air and hot gases is discharged from the burner into fracture 22 to ignite the formation oil around fracture 22.

The operation of burner 28 is continued for a short time after the ignition of the oil until a stable combustion front 46 is established in formation 14 around fracture 22. The advance of combustion front 46 away from fracture 22 leaves a zone 4-8 of heated formation rock around fracture 22. After combustion front 46 is established, burner 28 and fuel tubing 30 are removed from the well as is indicated in FIGURE 2.

To support in situ combustion of the formation oil, an oxygen-containing gas is injected downwardly through tubing 26 and annulus 42 into fracture 22 and formation 14. In this application for Letters Patent, the term oxygen-containing gas is used to describe a gas, such as air, that contains free oxygen; that is, oxygen that is not in chemical combination with other elements and is capable of supporting combustion of the oil. Although the particular in situ combustion method described in the drawings US. Patent No. 3,121,464

employs a propped fracture 22 and a single-point of entry from the well bore through circumferential notch 20, certain other in situ combustion techniques can be employed with the process of this invention, depending on their suitability as determined by the characteristics of the formation and of the crude oil. For example, in a relatively thin formation, the well bore might be cased only to the top of the formation and the oxygen-containing gas is then injected over the entire interval of the formation exposed in the well bore. As an alternative technique in thicker formations, the well bore is cased through the formation and the casing is perforated through a pre-selected interval of the formation. In such methods, the formation might or might not be fractured, depending upon the flow capacity of the formation, the required injection pressure, and other such factors.

The method of in situ combustion of oil around a highcapacity fracture, as described herein, is selected as a preferred method for use with the process of this invention because it offers several pertinent advantages over other combustion techniques. The location and propagation of combustion front 46 can be more easily controlled, and

. the formation can be heated more rapidly and more uniformly, when the combustion of the oil is restricted to a region of formation 14 around fracture 22. In addition, the high capacity, substantially horizontal fracture 22 serves as a lateral extension of well bore and the injected oxygen-containing gas flows through the fracture faces upwardly into formation 14 as indicated in FIG- URE 2. The hot gases flowing upwardly from combustion front 46 vaporize the volatile constituents in the oil above combustion front 46 and transport them into the unheated regions of formation 14 above fracture 22 where they condense and fall back into combustion front 46. Thus, convection currents induced in the oil above combustion front 46 cause a reflux action that results in a substantially constant supply of fuel o-t combustion front 46 around fracture 22.

After formation 14 has been heated sufliciently, the injection of oxygen-containing gas is stopped. Then in situ combustion is terminated and the possibility of down-hole explosions or combustion of subsequently produced oil is prevented according to the process of this invention by the injection of an oxygen-free gas downwardly through tubing 26 and annulus 42, into fracture 22 and formation 14. The term oxygen-free gas is used herein to describe a gas that does not contain free oxygen capable of supporting combustion of the formation oil, although such gas might contain oxygen in chemical combination with other elements. The oxygen-free gas displaces the oxygencontaining gas from well bore 10, fracture 22 and heated zone 48 through combustion front 46, where the oxygencontaining gas is consumed, thus leaving formation 14 and well bore 10 devoid of combustion-supporting gases. Suitable oxygen-free gases for use with this invention to prevent downhole combustion of produced oil are inert gases such as nitrogen and carbon dioxide. Natural gas is a preferred oxygen-free gas because it is ordinarily readily available and relatively inexpensive.

The volume of oxygen-free gas required to assure pre- Vention of down-hole combustion of the produced oil can be determined from data pertaining to the volume of the well bore and the volume of heated zone 48 in formation 14. Testing of formation rock and oil samples indicates the relationship between the volume of formation that is heated as a result of combustion of the crude oil in a cubic foot of oxygen-containing gas at formation conditions of temperature and pressure. Consequently, the volume of heated zone 48 can be calculated from the total volume of oxygen-containing gas injected.

The minimum suitable volume of oxygen-free gas injected is equal to the combined volumes of well bore 10 and the pore volume of heated zone 48. Preferably, the volume of oxygen-free gas injected is equal to the volume of well bore 10 plus two or three times the pore volum of heated zone 48. Injection of the preferred volume of oxygen-free gas assures that the combustion supporting gases in well bore 10 and formation 14 are consumed at combustion front 46 and are completely replaced by the injected oxygen-free gas.

After well bore 10 and formation 14 are cleared of oxygen-containing gases, the formation oil is produced through heated zone 48 and fracture 22 to well bore 10. The heated oil travels upwardly through tubing 26 and is conducted at the surface 12 to treating or storage facilities, not shown in the drawings. In some instances, the temperature of the produced oil might be such as to create problems in handling the oil at the surface. In other instances, for example when the minimum suitable volume of oxygen-free gas is injected to replace the combustionsupporting gases, or where formation heterogeneities interfere with uniform displacement of the oxygen-containing gas, it might be desirable to cool the oil below its ignition temperature after the oil flows out of formation 14 into well bore 10 in the event that an isolated pocket of oxygen-containing gas was not completely displaced through combustion front 46 by the oxygen-free gas. The produced oil can be cooled as it enters the well bore from the formation by injecting water downwardly through annulus 42. The cooling water mingles with the heated oil and is produced upwardly with the oil through tubing 26 to a separating apparatus not shown in the drawings. Such cooling of the oil might be desirable during the early stages of oil production, and the rate of water injection is regulated so as not to restrict unduly the rate of oil production through fracture 22.

Occasionally, when the process of this invention is applied for the thermal stimulation of a well that has been producing for some time, there is a danger of down-hole combustion during the operation of burner 28 owing to the accumulation in the well bore of parafl'ins deposited therein during the previous period of oil production. In such circumstances, down-hole fires during burner operation can be avoided by dumping a slug of solvent into the well bore to dissolve the paraffin accumulations therein prior to the injection of air to establish gas permeability in the formation. Then the subsequently injected air displaces the spent solvent into the formation prior to the injection of fuel and the ignition of the burner. Suitable solvents for use with this invention are those such as kerosene, lease crude, and mixtures of oil and surfactants or of water and surfactants.

The process of this invention is further explained by reference to its use in a well bore penetrating a formation extending from a depth of 3125 feet to 3305 feet. The formation has a porosity of 23 percent and a permeability of 78 millidarcys. The formation oil has a gravity of 42 API and is subject to plugging the formation and the well bore owing to deposition of paraffins from the oil during production. Tests indicate that parafiin deposition from the oil occurs at temperatures of F. or less, and it is therefore desirable to maintain the temperature of the oil above 105 F.

The well is cased with 5 /2 inch casing to the total depth of the well; namely, 3312 feet. A horizontal propped fracture is created in the formation at a depth of 3290 feet by the single-point entry technique. The fracture has a radius of feet from the well bore and a flow capacity of 30,000 millidarcy feet. Next the well bore apparatus is flushed with 30 barrels of kerosene to remove any possible parafiin accumulation. A string of 2 /2 inch tubing is run in the well bore to a depth of 3285 feet and a fluid fuel burner is seated in a tubing collar on the lower end of the tubing string. Then air is injected into the well bore and burner apparatus to displace the kerosene into the formation and to establish gas permeability in the formation around the fracture. A substantially constant rate of air in ection through the tubing and the casing is established at a rate of 900,000 s.c.f./day at a stabilized injection pressure of 1600 p.s.i.g. The burner is ignited and operated at an exhaust temperature of 800 F. using a fuel rate of eight gal/hr. of propane and injecting through the tubing 430 percent excess air as compared to stoichiometric conditions while additional air is injected through the casing. The burner thereby delivers heat at a rate of 17.5 million Btu/day.

After ignition of the oil in situ, fuel injection is stopped, the burner is removed from the well bore, and a total volume of air equal to 9.94 million s.c.f. is injected into the formation to sustain a combustion front in the oil around the fracture. A total quantity of heat of approximately one billion B.t.u. is transferred to the formation and raises the temperature of the heated zone around the fracture sufficiently to prevent paraflin deposition in the formation for a period of eight to ten months. Then 117,000 s.c.f. of natural gas is injected into the well bore and formation, that volume representing approximately twice the combined volume of the well bore and the pore volume of the heated zone. Subsequently, oil production is commenced and water is circulated in the well bore I for a period of 12 hours.

The preceding description of this invention discloses a method for heating a formation around a well bore by in situ combustion of oil contained in the formation whereby the danger of down hole combustion or explosion of subsequently produced oil is precluded. According to the process of this invention, the periodic thermal stimulation of a producing well by in situ combustion in a surrounding oil-bearing formation can be effected without endangering operating personnel or the well apparatus.

Therefore we claim:

1. A method of recovering oil containing paraflins from a subterranean rock formation by in situ combustion of the oil around a combination injection-production well bore having secured therein a string of easing extending through the formation, said method comprising cutting a circumferential ring-shaped notch in the casing and the wall of the well bore at a desired level in the formation, creating a fracture in the formation adjacent the notch, flushing the casing with a solvent to dissolve paraffin deposits accumulated in said casing, setting a string of tubing in the casing to a level slightly above the fracture, setting a fluid fuel burner in the tubing at the lower end thereof, injecting air downwardly through the casing and burner apparatus to displace said solvent from the well bore through the fracture into the formation and to establish gas permeability in the formation around the fracture, igniting the burner and injecting down the well bore and into the fracture a mixture of air and hot gases to ignite the formation oil, removing the burner while continuing the injection of air, thereby maintaining a combustion front in the formation oil around the fracture, discontinuing the injection of air and injecting an oxygen-free gas down the well bore and into the fracture to displace the air from the well bore and the formation around the fracture into the combustion front, and producing the formation oil through the fracture and upwardly through the well bore.

2. A method according to claim 1 wherein the solvent is kerosene.

3. In a method of recovering oil from a subsurface oilbearing formation in which the formation is heated by igniting oil in the formation adjacent the well, and an oxygen-containing gas is displaced down the well and into the formation to move the combustion front outwardly from the well into the formation, the improvement comprising, stopping the injection of oxygen-containing gas, injecting oxygen-free gas down the well bore to displace substantially completely the oxygen-containing gas from the well bore and through the formation to a point beyond the combustion front, and thereafter producing formation fluids through the formation into the well bore.

4. A method of heating, to increase the oil productior rate, a subterranean oil-bearing formation penetrated b3 a well bore comprising igniting the formation oil arounc the well bore, injecting an oxygen-containing gas down the well bore and into the formation to support combustion it the formation and move a combustion front outwardly from the well bore into the formation, terminating the injecting of oxygen'containing gas, thereafter injecting an oxygen-free gas down the well bore and into the formation to displace the oxygen-containing gas from the well and beyond the combustion front, and thereafter producing formation fluids through the formation into the well bore.

5. A method according to claim 4 wherein the oxygenfree gas in an inert gas.

6. A method according to claim 4 wherein the combustion front is propagated to at least 50 feet from the well bore before the injection of oxygen-containing gas is terminated.

7. A method according to claim 4 wherein the combustion front is propagated to between .50 to feet from the Well bore before the injection of oxygen-containing gas is terminated.

8. A method according to claim 4 wherein a fracture is created in the oil-bearing formation prior to ignition of the formation oil around the well bore.

9. A method according to claim 4 wherein water is injected down the tubing and annulus formed between the tubing and the production casing and/or well bore, thereby removing from the well bore any entrapped oxygen-containing gas.

10. A method according to claim 4 wherein the oxygenfree gas is natural gas.

11. A method according to claim 4 wherein the oxygen-free gas is nitrogen.

12. A method according to claim 4 wherein the oxygen-free gas is carbon dioxide.

13. In a method of recovering oil from a subterranean oil-bearing formation by in situ combustion of the oil around a combination injection-production well bore having secured therein a string of casing encompassing a string of tubing, thereby forming an annular space between said casing and said tubing, in which the formation is heated by igniting oil in the formation adjacent the well, and an oxygen-containing gas is displaced down the well and into the formation to move the combustion front outwardly from the well into the formation, the improvement comprising, stopping the injection of oxygen-com taining gas, injecting oxygen-free gas down the well bore and the annular space between the casing and tubing to displace substantially completely the oxygen-containing gas from the well bore and through the formation to a point beyond the combustion front, and thereafter producing formation fluids through the formation into the Well bore.

References Cited UNITED STATES PATENTS 2,670,047 2/1954 Mayes et a1. 166-11 2,880,802 4/1959 Carpenter 166-11 2,901,043 8/1959 Campion et a1. 166-11 2,906,340 9/1959 Herzog 166-39 3,013,609 12/1961 Ten Brink 166-39 3,121,464 2/1964 Huitt et a1. 166-421 3,160,208 12/1964 Jorda 166-11 X 3,171,482 3/1965 Simm 166-39 3,202,219 8/1965 Parker 166-11 X 3,211,221 10/1965 Huitt 166-421 CHARLES E. OCONNELL, Primary Examiner. STEPHEN I. NOVOSAD, Examiner. 

3. IN A METHOD OF RECOVERING OIL FROM A SUBSURFACE OILBEARING FORMATION IN WHICH THE FORMATION IS HEATED BY IGNITING OIL IN THE FORMATION ADJACENT THE WELL, AND AN OXYGEN-CONTAINING GAS IS DISPLACED DOWN THE WELL AND INTO THE FORMATION TO MOVE THE COMBUSTION FRONT OUTWARDLY FROM THE WELL INTO THE FORMATION, THE IMPROVEMENT COMPRISING, STOPPING THE INJECTION OF OXYGEN-CONTAINING GAS, INJECTING OXYGEN-FREE GAS DOWN THE WELL BORE TO DISPLACE SUBSTANTIALLY COMPLETELY THE OXYGEN-CONTAINING GAS FROM THE WELL BORE AND THROUGH THE FORMATION TO A POINT BEYOND THE COMBUSTION FRONT, AND THEREAFTER PRODUCING FORMATION FLUIDS THROUGH THE FORMATION INTO THE WELL BORE. 